bug#19972: Font size change doesn't update (window-total-width)

[Steve Purcell]

- Consider the case of full-screen frames, which never change in pixel size. 
The window-width should change when the font size changes in this case too.
- set-frame-font is a built-in function with a KEEP-SIZE param which tells it 
to try to preserve the pixel-size of the frame after the font has changed: my 
code is copied from there.

So there really shouldn’t be any assumption that the frame size will 
necessarily change when the font changes.

-Steve